1. Field of the Invention
The present invention relates to a gas sensor including a protector protecting a detecting element, which is exposed to a gas to be detected, from water.
2. Description of the Related Art
A gas sensor, such as an oxygen sensor, an NOX sensor, or an HC sensor, which detects a specific gas contained in an exhaust gas of an automobile or the like, is known in the art. An oxygen sensor, which is a typical gas sensor, includes a detecting element that includes a bottomed cylindrical solid electrolyte body made of a ceramic such as zirconia. A pair of electrodes is formed on the surfaces of the solid electrolyte body, the solid electrolyte body being interposed between the electrodes. When the gas sensor is used, a detecting electrode formed on the outer surface of the solid electrolyte body is exposed to the exhaust gas and a reference electrode formed on the inner surface of the solid electrolyte body is exposed to a reference gas (commonly, air). The detecting element detects oxygen contained in the exhaust gas by an electromotive force generated between the electrodes according to a difference in partial pressure of oxygen between two atmospheres isolated from each other by the solid electrolyte body, that is, between the exhaust gas and the reference gas.
Since this detecting element is not activated when the temperature is low, a heater heating the detecting element is provided near the detecting element. A heat generating resistor made of high melting point metal, such as tungsten or molybdenum, embedded in an insulating ceramic base body such as alumina, has been widely used as the heater. Since the heater is inserted into a cylindrical hole of the detecting element in use, the heater is formed in the shape of a round bar and the heat generating resistor is embedded into the front end portion of the heater. Further, electrode pads, which are used to supply current to the heat generating resistor, are disposed so as to be exposed to the outer surface of a rear end portion of the heater. Connection terminals, which are used to supply the current, are bonded to the electrode pads (for example, see JP-A-11-230930).
Furthermore, if moisture (water droplets) contained in an exhaust gas adheres to the detecting element (where the moisture, namely, water droplets contained in an exhaust gas wets the detecting element) when the temperature of the detecting element is high, there is a concern that cracks or breakage may occur on the detecting element due to thermal shock. For this reason, a protector covering the detecting element is mounted on the gas sensor, so that the detecting element is protected from water. For example, in each of gas sensors disclosed in JP-A-11-230930 and JP-A-2008-96419, a protector has a double structure that includes an inner protector covering a detecting element and an outer protector covering the inner protector. Outer introduction holes through which an exhaust gas is introduced into the outer protector are formed in the outer protector, and inner introduction holes where the exhaust gas introduced into the outer protector is introduced into a gas detecting chamber to which the detecting element is exposed are formed in the inner protector.
JP-A-2008-175685 is an example of the above-described related art.
3. Problems to be Solved by the Invention
However, in the gas sensor disclosed in JP-A-11-230930 or JP-A-2008-96419, the protector has a double structure that includes the outer protector and the inner protector. For this reason, the structure of the protector is complicated, so that many man-hours for assembly are required. Moreover, the cost of the protector is also increased.